Fluorinated Tetrathiafulvalenes with Preserved Electron-Donor

Sep 1, 2000 - Evaluation of the interchangeability of C–H and C–F groups: insights from crystal packing in a series of isomeric fluorinated benzan...
0 downloads 7 Views 283KB Size
Download PDF
J. Org. Chem. 2000, 65, 6479-6486

6479

Fluorinated Tetrathiafulvalenes with Preserved Electron-Donor Properties and Segregated Fluorous Bilayer Structures Based on F‚‚‚F Nonbonded Interactions† O. J. Dautel and M. Fourmigue´* “Sciences Mole´ culaires aux Interfaces”, FRE 2068 CNRS-Universite´ de Nantes, 2, rue de la Houssinie` re, BP32229, 44322 Nantes Cedex 3, France [email protected] Received April 5, 2000 (Revised Manuscript Received June 27, 2000)

The synthesis of the fluorinated 4,5-(2,2′-difluoropropylenedithio)-1,3-dithiol-2-one heterocycle with DAST allows for the preparation of several symmetrical and unsymmetrical di- and tetrafluorosubstituted tetrathiafulvalenes with accessible oxidation potentials (0.6 < E11/2 < 0.85 V vs SCE) despite the presence of the electron-withdrawing CF2 groups. The flexibility of the fluorinated sevenmembered ring identified from room-temperature NMR data has been thoroughly investigated by temperature-variable 1H and 19F NMR experiments, allowing for the identification of two independent folding processes, whose coalescence temperatures (Tc) and activation energies (∆Gq) were determined (Tc ) -15 °C, ∆Gq ) 50.2 kJ mol-1 and Tc ) 47 °C, ∆Gq ) 51.1 kJ mol-1). The analysis of the X-ray crystal structures of three of those fluorinated TTF demonstrates the efficiency of the nonbonded fluorine exclusion interactions for the stabilization of layered structures with fluorous bilayers, together with S‚‚‚S van der Waals interactions and C-H‚‚‚F hydrogen bonds. Introduction Fluorine/fluorine interactions play an unique role precisely because they are very weak if not negligible. This behavior originates in the low polarizability of the fluorine atom that leads to limited attractive interatomic dispersion forces.1 The consequences of these low attractive forces have been studied in the liquid phase, fluorocarbons have been indeed described as the least associated liquids,2 comparable in that respect to gases, which actually dissolve extremely well in them. This property has been exploited for applications in the biomedical field, such as oxygen transport and liquid ventilation.2 Amphiphilic molecules which incorporate both aliphatic and fluorinated fragments have been shown to organize into thermotropic smectic phases,3 lyotropic micellar systems2,4,5 or bilayers6 where the two types of fragments segregate. In the solid state, the absence of F‚‚‚F interactions, analyzed by Desiraju7 in the structures of fluorinated aromatics, leads to a segregation of the fluorinated and aromatics moieties in -SO2CF3-substituted aromatics.8 We want to take advantage of this effect, essentially observed in fluid phases, as a structure-orienting tool in †

Dedicated to Prof. Fred Wudl on the occasion of his 60th birthday. (1) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960; p 82. (2) Riess, J. G. New J. Chem. 1995, 19, 891 and references therein (3) (a) Ho¨pken, J.; Mo¨ller, M. Macromolecules 1992, 25, 2482. (b) Pensec, S.; Tournilhac, F.-G.; Bassoul P.; Durliat, C. J. Phys. Chem. B 1998, 102, 52. (4) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797. (5) Sadtler, V. M.; Giulieri, F.; Krafft, M.-P.; Riess, J. G. Chem. Eur. J. 1998, 4, 1952. (6) (a) Rabolt, J. F.; Russel, T. P.; Twieg, R. J. Macromolecules 1984, 17, 2786. (b) Russel, T. J.; Rabolt, J. F.; Twieg, R. J.; Siemens, R. L.; Farmer, B. L. Macromolecules 1986, 19, 1135. (7) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 8725. (8) Wolff, J.; J. Gredel, F.; Oeser; T.; Irngartinger, H.; Pritzkow, H. Chem. Eur. J. 1999, 5, 29.

the control of the solid-state architectures of fluorinated molecules, for the rational construction of columnar or layered structures and particularly for the elaboration of conducting or magnetic molecular solids of controlled dimensionality. We also postulate that a limited number of such fluorine atoms, adequately located in the molecular framework, would be sufficient to induce a strong structural effect on the self-assembly properties of those molecules in the solid state. We describe in this paper the synthesis of the fluorinated 4,5-(2,2′-difluoropropylenedithio)-1,3-dithiole-2-one heterocycle, as a precursor to a variety of different symmetrical and unsymmetrical di- and tetrafluorosubstituted tetrathiafulvalenes with accessible oxidation potentials. The flexibility of the fluorinated sevenmembered ring identified from room-temperature NMR data has been thoroughly investigated by temperaturevariable 1H and 19F NMR experiments, allowing for the identification of two independent folding processes, whose coalescence temperatures (Tc) and activation energies (∆Gq) were determined. The analysis of the X-ray crystal structures of three of those fluorinated TTF will demonstrate the efficiency of the nonbonded fluorine exclusion interactions in the control of the solid state arrangement, in addition to S‚‚‚S van der Waals interactions and C-H‚‚‚F hydrogen bonds. Results and Discussion Syntheses. Several tetrathiafulvalenes bearing fluorinated moieties have been described, as the perfluorodibenzotetrathiafulvalene9 or the tetrakis(trifluoromethyl)tetrathiafulvalene,10,11 which both exhibit a poor donor ability due to the strong electron-withdrawing (9) Knight, B. W.; Purrington, S. T.; Bereman, R. D.; Singh, P. Synthesis 1994, 460.

10.1021/jo000518l CCC: $19.00 © 2000 American Chemical Society Published on Web 09/01/2000

6480

J. Org. Chem., Vol. 65, No. 20, 2000 Scheme 1

Dautel and Fourmigue´ Scheme 2

Scheme 3

effect of the fluorinated moieties directly conjugated with the TTF. As a consequence, cation radical salts derived from these molecules were never reported. Other examples involve highly soluble TTF cores functionalized with long, flexible C8F17-perfluoroalkyl chains12 for the elaboration of Langmuir-Blodgett films.13 Looking for alternative substitution patterns that would (i) preserve the donor ability of the π-redox TTF core and (ii) allows for a limited flexibility for good crystallization properties, we envisioned the original tetrathiafulvalene molecules 5, 8, and 11, which meet these requirements. The specificity of these symmetrically or unsymmetrically substituted TTFs lies in the presence of only one or two CF2 group in a seven-membered ring on one or both sides of the molecule, and isolated from the π-redox active core by the CH2-S spacers. The key step for the preparation of 5, 8, and 11 lies in the successful fluorination of the ketonic group of 1 or 2 by reaction with DAST (Scheme 1).14,15 Note the chemoselectivity of this reaction which does not affect the oxygen atom of the dithiocarbonate group in 2. Indeed, the lowered electrophilicity of the carbonyl group of the dithiocarbonate inhibits the transfer of the nucleophilic fluorine, as also observed with esters.16 The P(OMe)3-assisted coupling17 of the fluorinated dithio- or trithiocarbonate 3 or 4 afforded the symmetrically substituted TTF 5 in 23% yield from 3, in 91% yield from 4. The preparation of the unsymmetrically substituted analogues 8 and 11 involves the crosscoupling of the trithiocarbonate 3 with either the diester 618 or with 4,5-bis(2-cyanoethylthio)-1,3-dithiole-2-one 9.19 From 3 and 6 (Scheme 2), the fluorinated TTF bearing (10) (a) Hartzler, H. D. J. Am. Chem. Soc. 1970, 92, 1412; (b) 1973, 95, 4379. (c) Bianchini, C.; Meli, A. J. Chem. Soc., Chem. Commun. 1983, 1309. (d) Mu¨ller, H.; Lerf, A.; Fritz, H. P. Liebigs Ann. Chem. 1991, 395. (11) (a) Engler, E. M.; Patel, V. V.; Schumaker, R. R. J. Chem. Soc., Chem. Commun. 1979, 516. (b) Schumaker, R. R.; Engler, E. M. J. Am. Chem. Soc. 1980, 102, 6652. (12) Nozdryn, T.; Favard, J.-F.; Cousseau, J.; Jubault, M.; Gorgues, A.; Orduna, J.; Garı´n, J. J. Fluorine Chem. 1997, 86, 177. (13) Dupart, E.; Agricole, B.; Ravaine, S.; Mingotaud, C.; Fichet, O.; Delhae`s, P.; Ohnuki, H.; Munger, G.; Leblanc, R. M. Thin Solid Films 1994, 243, 575. (14) Marshallsay, G. J.; Bryce, M. R.; Cooke, G.; Jørgensen, T.; Becher, J.; Reynolds, C. D.; Wood, S. Tetrahedron 1993, 49, 6849. (15) Middleton, W. J. J. Org. Chem. 1975, 40, 574. (16) Brunnelle, W. H.; McKinnis, B. R.; Narayanan B. A. J. Org. Chem. 1990, 55, 768. (17) Reviews: (a) Bryce, M. R. Aldrichim. Acta 1985, 18, 73. (b) Krief, A. Tetrahedron 1986, 42, 1209.

one CF2 group and two ester groups was obtained in good yield after chromatographic separation from the symmetrical coupling products. Decarboxylation20 with LiBr/ HMPA occurred in two steps, one CO2Me group is removed at 80 °C, the second one requires heating to 150 °C, affording the CF2-derivatized TTF 8 in excellent yield. Similarly, coupling of 3 with 919 (Scheme 3) afforded the unsymmetrically substituted TTF 10 in good yield. The 2-cyanoethyl moieties in 10 act here as protecting groups for the thiolate groups and can be displaced by action of a strong base such as EtONa. Reaction of the corresponding fluorinated TTF dithiolene with 1,2-dibromoethane afforded the corresponding functionalized TTF 11 in excellent yield. In the course of this work, a second route was also developed for the preparation of the EDT-TTF derivative 11, which involves the DAST fluorination in the last step for economical reasons given the high price of the DAST reagent (Scheme 4). For that purpose, the ketonic group in 2 was first protected as an acetal 12. This reaction, described by Marshallsay et al. in 1993,14 was optimized here by using p-toluenesulfonic acid mono hydrate instead of sulfuric acid as acid catalyst. P(OMe)3assisted cross coupling of 12 with the dithiocarbonate 13 afforded the acetal-derivatized TTF 14 after chromatographic separation from the symmetrical coupling products. Hydrolysis of 14 regenerated the ketonic group for DAST fluorination of 15. This last step however proved sluggish despite prolonged gentle heating of the reaction (18) Easton, D. B.; Leaver, D. J. Chem. Soc., Chem. Commun. 1965, 22, 585. (19) Sventrup, N.; Rasmussen, K. M.; Hansen, T. K.; Becher, J. Synthesis 1994, 8, 809. (20) Lakshmikantham, M. V.; Cava, M. P. J. Org. Chem. 1976, 41, 882.

Fluorinated Tetrathiafulvalenes

J. Org. Chem., Vol. 65, No. 20, 2000 6481

Scheme 4

mixture and only 32% of the desired TTF was obtained by this route. Two Independent Folding Processes Identified by Variable-Temperature 1H and 19F NMR Studies. The fluorine atoms of all these fluorinated molecules were not observed by 19F NMR at room temperature where only very broad nearly indistinguishable massifs were obtained. The fluxional behavior of the seven-membered ring incorporating the difluoromethylene moiety was confirmed by low-temperature (-30 °C) NMR experiments that allowed for the observation of the 19F NMR spectra. Under those conditions, however, most of the molecules described here precipitated in the NMR tubes. Therefore, a complete NMR study of this fluxional behavior has been performed only on the most soluble compound (4), in CDCl3 at low temperatures (e10 °C) and in C6D6 at higher temperatures (g10 °C). As shown in Figure 1a, the 1H NMR spectrum of 4 shows two multiplets at δ 2.75 and 3.15 suggesting the presence of two types of hydrogen nuclei (2Ha and 2He). In the same way, the 19F NMR spectrum (Figure 1b) shows two doublets of triplets of triplets at δ -77 and -103.5 suggesting the presence of two different fluorine atoms (Fe and Fa). We can reasonably consider that, at this temperature, the dithiocarbonate 4 is in its most stable configuration, that is to say a chair form (Figure 2). The attribution of the different signals was based on the fluorine atom Fa, which is expected to be shielded by the lone pairs of the neighboring sulfur atom. So its signal was considered to be at -103 ppm. This hypothesis was supported by the analysis of its heteronuclear coupling constants (3JFH) considering the Karplus21 rules that link the 3J coupling constants with the dihedral angle (Φ). According to those rules, Jcis (Φ ) 60°) should be less than Jtrans (Φ ) 180°). So we expect here JFaHa to be larger than JFaHe, JFeHa, and JFeHe. The signal attributed to Fa (δ -103.5) follows these rules with a doublet (2JFaFe ) 247 Hz) of triplets (3JFaHa ) 31 Hz) of triplets (3JFaHe ) 7 Hz) with indeed 3JFaHe < 3JFaHa. The 19F signal positioned at lower field (δ -77) is attributed to Fe with a doublet (2JFeFa ) 247 Hz) of triplets (3JFeHe ) 10 Hz) while 3JFeHa is probably too small to be seen. A similar analysis performed on the 1H NMR spectrum drives us to conclude that the signal located at δ 2.75 must be attributed at the two axial protons Ha with the large 3JHF coupling constant (3JFaHa ) 31 Hz) observed in this multiplet while the signal located at δ 3.15 must be attributed to the two equatorial protons He with smaller 3JHF coupling constants. (21) Karplus, M. J. Am. Chem. Soc. 1963, 85, 2870.

Figure 1. 1H (a) and CDCl3 at -30 °C.

19F

(b) NMR spectra of compound 4 in

Figure 2. Left: the most stable conformation of 4. Right: a Newman representation of 4 with the dihedral angles between the different nuclei Fa, Fe, Ha, and He (a and e stand for axial and equatorial, respectively).

The variable-temperature 1H and 19F NMR studies showed two independent folding processes. The first one is observed at low temperature (Tc ) -15 °C) on the 1H NMR spectra (Figure 3) where the two massifs attributed to He and Ha coalesce upon warming from -30 °C, while in the 19F spectra (Figure 4), a broadening of the peaks reflects the equivalence of the two types of hydrogen nuclei. This low-temperature folding process can therefore be attributed to an inversion around the C3-C5 axe (Figure 5). The activation energy for this fluxional behavior is estimated at 50.2 kJ mol-1 from the Eyring equation.22 The second process, which occurs at higher temperature, is observed on the 19F NMR spectra (Figure 4). Both broad room-temperature singlets attributed to Fa and Fe coalesce upon warming in a broad singlet above (22) Eyring, H. Chem. Rev. 1935, 17, 65.

6482

J. Org. Chem., Vol. 65, No. 20, 2000

Dautel and Fourmigue´

Figure 5. The two folding processes observed in 4.

Figure 3. Variable-temperature 1H NMR for compound 4 in CDCl3.

Figure 6. View of the layered structure of 5 along the c axis showing the fluorine segregation. Table 1. Geometrical Characteristics of the CH‚‚‚F Hydrogen Bonds in 5, 8, and 11 compd

interaction

H‚‚‚F (Å)

C‚‚‚F (Å)

C-H‚‚‚F (°)

5

C4H4A‚‚‚F12 C4H4A‚‚‚F22 C4H4B‚‚‚F13 C6H6A‚‚‚F13 C6H6B‚‚‚F12 C6H6B‚‚‚F22 Csp21H1‚‚‚F22 Csp22H2‚‚‚F12 C9H9B‚‚‚F12 C4H4B‚‚‚F12 C9H9B‚‚‚F22 C10H10B‚‚‚F22 C10H10A‚‚‚F22

2.958 2.702 2.741 2.612 2.491 2.692 2.542 2.800 2.759 2.362 2.529 2.691 2.779

3.794 3.512 3.412 3.304 3.408 3.473 3.314 3.653 3.459 3.145 3.221 3.553 3.211

144.9 141.4 126.8 128.5 157.5 137.8 140.6 152.8 129.6 137.3 128.3 148.3 107.7

8 11

Figure 4. Variable-temperature 19F NMR for compound 4 in C6D6.

Tc ) 47 °C (∆Gq ) 51.1 kJ mol-1) where the two fluorine nuclei become magnetically undistinguishable. As a consequence, the 1H NMR spectra (Figure 3) change from a ABX4 system at room temperature to a A2X4 one above 47 °C. This second process corresponds to an inversion around the S1-S4 axis (Figure 5). Fluorine Segregation Controls the Structures of the Neutral Donor Molecules. The symmetrically substituted tetrathiafulvalene 5 crystallizes as thin red plates from toluene in the monoclinic system, space group P21/c. As shown in Figure 6, a full segregation of the heteroaromatic and the fluorinated moieties is observed, giving rise to a unique layered structure with stacks of molecules and a compact fluorinated bilayer: the firstneighbor F‚‚‚F distances vary between 2.769(3) and 3.254(3) Å, values around the van der Waals F‚‚‚F

distance (2.94 Å).23 This arrangement results from the interplay between (i) the S‚‚‚S van der Waals interactions which stabilize the face-to-face stacking of the TTF cores along the b axis and the lateral arrangement of these stacks along the c direction and (ii) the striking full segregation of the CF2 groups on the outer ends of these layers. In addition, short C-H‚‚‚F distances within a layer and through the F‚‚‚F van der Waals gap are also observed (Table 1), with geometrical features comparable to those reported by Desiraju24 in p-difluorobenzene (dH‚ 25 26 ‚‚F ) 2.49 Å). These C-H‚‚‚Hal hydrogen bonds, albeit weak in essence, have been shown to play a crucial role in the organic solid state of halogenated molecules, and despite the fact that the fluorine atoms are poor hydrogen (23) Bondi, A. J. Phys. Chem. 1964, 68, 441. (24) Thalladi, V. R.; Weiss, H.-C.; Bla¨ser, D.; Boese, R.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702.

Fluorinated Tetrathiafulvalenes

J. Org. Chem., Vol. 65, No. 20, 2000 6483

Figure 7. View of the unit cell of the nonfluorinated analogue of 5, the bis(propylenedithio)tetrathiafulvalene with the usual orthogonal inversion-centered dimers.

bond acceptors, the hydrogen bond donor character of the methylenic moieties in 5 is most probably enhanced here by the presence of the neighboring electron-withdrawing CF2 entities. An enlightening comparison can be made with the structure of the nonfluorinated analogue, the bis(propylenedithio)tetrathiafulvalene (BPDT-TTF), which crystallizes into orthogonal inversion-centered dimers (Figure 7),27 as most frequently observed with those sulfur-rich molecules such as EDT-TTF28 or BEDT-TTF,29 whose structures are mainly controlled by S‚‚‚S van der Waals interactions. On the other hand, in the structure of the tetrakis(trifluoromethyl)tetrathiafuvalene,30 the criss-cross arrangement of the molecules on top of each other gives rise to cylindrical stacks with all CF3 groups exposed to the surface through similar fluorine segregation effects. These columns then organize into a pseudohexagonal array reminiscent of thermotropic discotic phases. Thus in 5, the outer difluoromethylene groups are responsible for inducing an original layered solid state arrangement which is not usually observed with those molecules in their neutral, unoxidized state. Also this structure is reminiscent of that observed in smectic liquid crystalline phases. DSC measurements were performed with 5 in an attempt to observe such a behavior but only decomposition, at 306 °C, was observed upon heating. The X-ray structures of the unsymmetrically substituted fluorinated TTF 8 and 11 confirm those trends (25) Such C-H‚‚‚F interactions were also recently reported in the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salt of 2-fluorotetracyanoquinodimethane (F1TCNQ): Hasegawa, T.; Inukai, K.; Kagoshima, S.; Sugawara, T.; Mochida, T.; Sugiura, S.; Iwasa, Y. J. Chem. Soc. Chem. Commun. 1997, 1377. (26) Desiraju, G. R.; Steiner, T. In The Weak Hydrogen Bond; Oxford University Press: New York, 1999. (27) Porter, L. C.; Kini, A. M.; Williams, J. M. Acta Crystallogr. C 1987, 43, 998. (28) Garreau, B.; De Montauzon, D.; Cassoux, P.; Legros, J.-P.; Fabre, J.-M.; Saoud, K. Chakroune, S. New J. Chem. 1995, 19, 161. (29) (a) Kobayashi, H.; Kato, R.; Mori, T.; Ayashi, A.; Saki, Y.; Saito, G.; Inokuchi, H. Chem. Lett. 1983, 759. (b) Kobayashi, H.; Kato, R.; Kobayashi, A.; Mori, T.; Inokuchi, H. Solid State Commun. 1986, 40, 473. (30) Mono, S.; Pritzkow, H.; Sundermeyer, W. Chem. Ber. 1993, 126, 2111.

Figure 8. Top: a view of the unit cell of 8, showing the fluorine (in black) segregation and the layered structure. Bottom: detail of one organic slab, viewed along the long molecular axis of 8.

Figure 9. Top: a view of the unit cell of 11, showing the fluorine (in black) segregation and the layered structure. Bottom: detail of one organic slab, viewed along the long molecular axis of 11.

despite the presence of only one CF2 group. As shown in Figures 8 and 9, the donor molecules crystallize into inversion-related face-to face dimers, with an interplanar distance of 3.557(3) and 3.443(1) Å in 8 and 11, respectively (Table 2). Note that the molecules are strongly distorted from planarity; the dithiole rings of the central TTF core are indeed folded along the S- - -S axis by 21.29(6) and 12.0(1)° in 8, 20.18(5) and 19.20(5)° in 11. Those diads then organize in the solid state in a layered motif characterized by the same fluorine segregation observed with 5 and the presence of C-H‚‚‚F hydrogen bonds (Table 1) which involve not only the methylenic groups (-SCH2CF2-) of 8 and 11 but also the ethylenic hydrogen

6484

J. Org. Chem., Vol. 65, No. 20, 2000

Dautel and Fourmigue´

Table 2. Interplanar Distances and Folding Angles for 5, 8, and 11

compd

torsion angle β1 (deg)

torsion angle β2 (deg)

distance d (Å)

5 8 11

7.6(3) 21.29(6) 20.18(5)

7.6(3) 12.0(1) 19.20(5)

4.094(8) 3.557(3) 3.443(1)

Table 3. Oxidation Potentials of 5, 8, and 11 compd

solvent

E11/2a

E21/2a

5 BPDT-TTF 8 EDT-TTF 11 BEDT-TTF BPDT-TTF

THF THF CH3CN CH3CN CH3CN CH3CN CH3CN

0.87 0.74 0.48 0.44 0.58 0.56 0.54

1.07 0.92 0.82 0.75 0.83 0.80 0.81

a In V vs SCE; supporting electrolyte, 0.1 M Bu NPF , scan rate, 4 6 0.1 V s-1.

atoms of 8. Note that the organization of the molecular dimers within the slabs is reminiscent of the so-called κ phases of conducting and superconducting BEDT-TTF salts. This rare κ-phase organization in the structure of neutral donor molecules,31 characterized by the mean planes of contacting molecules being almost perpendicular and their principal axes essentially parallel, is all the more fortunate since it might be conserved in the cation radical salts of those fluorinated donor molecules. Electrochemical Properties. Cyclic voltammetry data for the new donor molecules are collected in Table 3 together with those of reference compounds TTF, BEDT-TTF, and BPDT-TTF. They all exhibit two reversible oxidation waves, as expected for TTF derivatives. The comparison of the first oxidation potential of 5, observed at 0.87 V vs SCE in THF,32 with its nonfluorinated analogue the bis(propylenedithio)tetrathiafulvalene (BPDT-TTF) (0.74 V vs SCE in THF) shows that the four fluorine atoms exert a small but noticeable anodic shift despite the CH2-S spacers of the sevenmembered rings. The same effect is observed to a less extend in 8 and 11, when compared with the non fluorinated EDT-TTF or BEDT-TTF analogues. Those values are also to be compared with those of (CF3)4TTF10,11 or perfluorodibenzotetrathiafulvalene9 which oxidize at much higher potentials in CH3CN, 1.05 and 1.45 V vs SCE, respectively, demonstrating the soundness of our approach. Accordingly, 5 could be successfully electrocrystallized in the presence of various electrolytes such as the n-Bu4N+ salts of the linear anions ICl2-, IBr2-, and I2Br-, affording, air-stable crystalline salts whose structures and magnetic properties will be described elsewhere. (31) One other example has been described: Batsanov, A. S.; Bryce, M. R.; Cooke, G.; Dhindsa, A. S.; Heaton, J. N.; Howard, J. A. K.; Moore, A. J.; Petty, M. C. Chem. Mater. 1994, 6, 1419. (32) The use of THF, for solubility reasons, in the characterization of 5 introduces an extra anodic shift when compared with measurements performed in CH3CN. See also: Lichtenberger, D. L.; Johnston, R. L.; Hinkelmann, K.; Suzuki, T.; Wudl, F. J. Am. Chem. Soc. 1990, 112, 3302.

Conclusion This work provides an easy access to a novel class of symmetrically as well as unsymmetrically substituted fluorinated TTFs where the CF2 groups are not conjugated with the TTF π redox core, as confirmed by the accessible oxidation potentials. The intermolecular F‚‚‚ F nonbonded interactions observed in the solid state, combined with C-H‚‚‚F hydrogen bonds, afford layered structures, an highly unusual organization motif for TTF molecules in their neutral state.

Experimental Section General Considerations. Unless otherwise indicated, all reactions were carried out under nitrogen. Solvents (dichloromethane, acetonitrile) were dried and freshly distilled over P2O5. Note that DAST and related dialkylaminosulfur trifluorides should not be heated since they are said to undergo catastrophic decomposition (explosion or detonation) with gas evolution upon heating above 90 °C.33 1H, 13C, and 19F NMR spectra were recorded at 200 (1H, 19F) and 50 MHz (13C). The chemical shifts are given in δ ppm, downfield from internal Me4Si for 1H and 13C NMR, upfield from external CFCl3 for 19F NMR. Mass spectra were taken in the EI mode with an ionization energy of 70 eV and a current intensity of 300 µA. Cyclic voltammetry were taken with a Pt disk (Ø ) 1 mm) as working electrode and a Pt wire as counter electrode and a SCE as reference electrode. Elemental analyses were performed either at the Service Central d’Analyse (CNRS), Vernaison, France or the Institut de Chimie des Substances Naturelles (CNRS), Gif/Yvette, France. 6,6-Difluoro-5,7-dihydro-5H-[1,3]dithiolo[4,5-b][1,4]dithiepin-2-thione (3). 1 (2 g, 7,94 mmol) was dissolved in dry CH2Cl2 (200 mL). A solution of DAST (1.16 mL, 8.7 mmol) in CH2Cl2 (4 mL) was added dropwise at 0 °C, and the mixture was allowed to warm to room temperature and stirred for 2 h. The reaction mixture was poured into ice-cold water (200 mL), the lower organic layer was separated, washed with water, and dried (MgSO4), and the solvent was evaporated to afford 2.1 g of crude product, which was purified by column chromatography (silica gel, dichloromethane) and recrystallization from acetonitrile to give orange needles of 3 (1.85 g, 85%): mp 192 °C (CH3CN); Rf ) 0.85 (CH2Cl2); IR (KBr) 1078, 1039 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.99 (4H, m); 13C NMR (CDCl3, 50 MHz) δ 38.13 (2C, t, J ) 30 Hz), 121.04 (2C, t, J ) 239 Hz), 137.80, 209.20; EIMS (m/z) 274 (M.+, 77), 198 (37), 88 (100). Anal. Calcd for C6H4F2S5 (274.425): C, 26.26; H, 1.47; F, 13.85; S, 58.42. Found: C, 26.31; H, 1.37; F, 13.51; S, 58.38. 6,6-Difluoro-5,7-dihydro-5H-[1,3]dithiolo[4,5-b][1,4]dithiepin-2-one (4). This compound (3.5 g, 80%) was prepared from 2 (4 g, 16.9 mmol) and DAST (2.48 mL, 18.6 mmol) as a white solid, employing the same procedure as described above for compound 3. Recrystallization from acetonitrile afforded 4 as gray needles: mp 183 °C (CH3CN); Rf ) 0.80 (CH2Cl2); IR (KBr) 1664, 1080 cm-1; 1H NMR (CDCl3, 200 MHz) δ 3.03 (4H, m); 13C NMR (CDCl3, 50 MHz) δ 38.35 (2C, t, J ) 29 Hz), 120.91 (2C, t, J ) 244 Hz), 131.26, 188.02; EIMS (m/z) 258 (M.+, 93), 230 (51), 154 (51), 88 (100). Anal. Calcd for C6H4F2O1S4 (258.358): C, 27.89; H, 1.56; F, 14.71; S, 49.65. Found: C, 27.93; H, 1.52; F, 14.28; S, 49.91. Bis(difluoropropylenedithio)tetrathiafulvalene (5). From 3. A mixture of 3 (0.1 g, 0.365 mmol) and freshly distilled P(OMe)3 (60 equiv) was refluxed for 1 h. The reaction mixture was allowed to cool to room temperature and an equal volume of methanol was added. The precipitate was filtered and washed with ethanol and diethyl ether to give the crude product as a red powder. Two recrystallizations from toluene afforded red plates of 5 (20 mg, 23%) (33) Messina, P. A.; Mange, K. C.; Middleton, W. J. J. Fluorine Chem. 1989, 42, 137.

Fluorinated Tetrathiafulvalenes From 4. A mixture of 4 (0.4 g, 1.55 mmol) and freshly distilled P(OMe)3 (60 equiv) was refluxed for 3 h. The reaction mixture was allowed to cool to room temperature, and an equal volume of methanol was added. The precipitate was filtered and washed with ethanol and diethyl ether to give the crude product as a red powder. Two recrystallizations from toluene afforded red plates of 5 (300 mg, 80%): mp 306 °C (toluene, dec); Rf ) 0.85 (CH2Cl2); IR (KBr) 1075 cm-1; 1H NMR (DMSO/ CS2, 200 MHz) δ 3.03 (8H, m); EIMS (m/z) 484 (M.+, 83), 286 (38),198 (42), 88 (100). Anal. Calcd for C12H8F4S8 (484.717): C, 29.74; H, 1.66; F, 15.68; S, 52.92. Found: C, 30.01; H, 1.64; F, 15.04; S, 52.74. 2-[1,3]Dithiol-2-ylidene-6,6-difluoro-5,7-dihydro5H-[1,3]dithiolo[1,4]dithiepin-4,5-dicarboxylic Acid Dimethyl Ester (7). A mixture of 3 (2 g, 7.3 mmol) and the diester 6 (1.71 g, 7.3 mmol) in freshly distilled P(OMe)3 (120 mL) was refluxed for 3 h. The reaction mixture was allowed to cool to room temperature. The precipitate was filtered, washed with ethanol and diethyl ether to give 0.59 g of F4BPDT-TTF 5 (20%) as a red microcrystalline powder. Concentration of the filtrate followed by column chromatography (silica gel, toluene) afforded 1.25 g of the diester 7. The resulting dark red solid can be further purified by recrystallization from toluene/cyclohexane (3/7) to give 7 (1.18 g, 35%) as dark red needles: mp 224 °C (toluene/cyclohexane 3:7); Rf ) 0.49 (toluene); IR (KBr) 1721, 1250, 1020 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.95 (4H, m), 3.84 (6H, s); EIMS (m/z) 460 (M+., 37), 198 (38), 88 (100). Anal. Calcd for C13H10F2O4S6 (460.613): C, 33.90; H, 2.19; F, 8.25; S, 41.77. Found: C, 34.06; H, 2.21; F, 7.92; S, 42.02. 2-[1,3]Dithiol-2-ylidene-6,6-difluoro-5,7-dihydro-5H[1,3]dithiolo[4,5-b][1,4] dithiepine (8). A suspension of lithium bromide (1.09 g, 11 equiv, 12.5 mmol) in hexamethylphosphoric triamide (30 mL) was stirred at 80 °C (LiBr dissolved at 60 °C). The diester 7 (500 mg, 1.09 mmol) was added, and the mixture was stirred for a further 20 min (the first decarboxylation occurred). The second decarboxylation was obtained by increasing the temperature to 150 °C and stirring the solution for 30 min. After being cooled at room temperature, the reaction mixture was diluted with ethyl acetate (250 mL), washed with brine (3 × 150 mL), dried over MgSO4, and concentrated. The residue was subjected to column chromatography with toluene (degassed with N2) as eluent. One recrystallization from acetonitrile afforded 8 as red crystals (340 mg, 91%): mp 191 °C (CH3CN); Rf ) 0.9 (toluene); IR (KBr) 1411, 1253, 1067 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.94 (4H, m), 6.34 (2H, s); 13C NMR (CDCl3, 50 MHz) δ 38.35 (2C, t, J ) 29 Hz), 105.50, 120.91 (2C, t, J ) 244 Hz), 131.77; EIMS (m/z) 344 (M+., 100), 146 (88), 102 (49), 88 (57). Anal. Calcd for C9H6F2S6 (344.539): C, 31.37; H, 1.76; F, 11.03; S, 55.84. Found: C, 31.51; H, 2.21; F, 10.83; S, 55.69. 2,3-Bis(2-cyanoethylthio)-5,7-(6,6-difluoropropylenedithio)tetrathiafulvalene (10). A mixture of 3 (2 g, 7.3 mmol) and 4,5-bis(2-cyanoethylthio)-1,3-dithiole-2-one 919 (2 g, 7.3 mmol) in P(OMe)3 (100 mL, freshly distilled) was refluxed for 6 h. The reaction mixture was allowed to cool to room temperature. The precipitate was filtered and washed with ethanol and diethyl ether to give 0.40 g of 5 (15%) as a red microcrystalline powder. Concentration of the filtrate followed by column chromatography (silica gel, CH2Cl2) afforded 1.6 g (43%) of the bis-cyanoethyl protected TTF 10. The resulting yellow solid can be further purified by recrystallization from acetonitrile to give 1.5 g (40%) of yellow platelets of 10: mp 191 °C (CH3CN); Rf ) 0.29 (CH2Cl2); IR (KBr) 2977, 2920, 2250, 1260, 1077 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.74 (t, 2H, J ) 6.8 Hz), 2.95 (4H, m), 3.09 (t, 2H, J ) 6.8 Hz); EIMS (m/z) 514 (M.+, 51), 286 (100), 166 (67), 88 (89). Anal. Calcd for C15H12F2N2S8 (514.799): C, 35.00; H, 2.35; F, 7.38; N, 5.44; S, 49.83. Found: C, 35.06; H, 2.31; F, 6.76; N, 5.21; S, 50.13. 2,3-(Ethylenedithio)-5,7-(6,6-difluoropropylenedithio)tetrathiafulvalene (11). From TTF 10. A solution of sodium methanolate (previously prepared from sodium (36 mg, 1.56 mmol, 4 equiv) in MeOH (3 mL) was added dropwise to a suspension of 10 (200 mg, 0.39 mmol) in degassed MeOH (25

J. Org. Chem., Vol. 65, No. 20, 2000 6485 mL) at room temperature. After being stirred for 6 h, the red limpid solution was treated with 2 equiv of 1,2-dibromoethane (69 µL, 0.8 mmol) and the reaction mixture stirred for 12 h. The precipitation of 11 was completed by cooling the reaction mixture at 0 °C. The precipitate was filtered and washed with cold EtOH (20 mL) and cold Et2O (20 mL). Purification by column chromatography (silica gel, CH2Cl2/cyclohexane) followed by recrystallization from toluene give 140 mg (82%) of orange platelets of 11. From 15. The TTF 15 (90 mg, 0.218 mmol) was dissolved in dry CH2Cl2 (150 mL) and cooled to 0 °C. A solution of DAST (32 µL, 0.243 mmol) in CH2Cl2 (2 mL) was added dropwise, and the mixture was carefully warmed to 35 °C and stirred for 24 h. The reaction mixture was poured into ice-cold water (150 mL), the lower organic layer was decanted, washed with water, and dried (MgSO4), and the solvent was evaporated to afford 50 mg of crude product, which was purified by column chromatography (silica gel, toluene) and recrystallization from toluene to give orange platelets of 11 (30 mg, 32%): mp 226 °C (toluene); Rf ) 0.58 (CH2Cl2/cyclohexane); IR (KBr) 1262, 1076 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.93 (4H, m), 3.29 (4H, s); EIMS (m/z) 434 (M+., 72), 406 (39), 166 (53), 88 (100). Anal. Calcd for C11H8F2S8 (434.709): C, 30.39; H, 1.85; F, 8.74; S, 59.01. Found: C, 30.51; H, 1.83; F, 8.50; S, 58.07. Ketal-Protected [1,3]Dithiolo[4,5-b][1,4]dithiepine-2,6dione (12). To a solution of 2 (4 g, 16.9 mmol) in dry toluene (500 mL) were added ethylene glycol (1.14 mL, 20.34 mmol) and TsOH‚H2O (0.161 g, 0.85 mmol). A Dean-Stark apparatus was assembled and the reaction refluxed with stirring for 4 days. The organic phase was washed with a solution of sodium hydrogen carbonate (2 × 200 mL), dried (MgSO4), and concentrated. The crude product was recrystallized from acetonitrile to afford white needles of 12 (4.57 g, 97%): mp 145 °C (CH3CN); Rf ) 0.59 (CH2Cl2); IR (KBr) 2993, 2896, 1660, 1610 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.83 (4H, s), 4.09 (4H, s); EIMS (m/z) 280 (M+., 40), 194 (100), 166 (47). Anal. Calcd for C8H8O3S4 (280.414): C, 34.27; H, 2.88; O, 17.12; S, 45.74. Found: C, 34.23; H, 2.91; O, 17.36; S, 45.71. Ketal-Protected 2,3-(Ethylenedithio)-5,7-(6-oxopropylenedithio)tetrathiafulvalene (14). A mixture of the ketal 12 (1 g, 3.60 mmol) and 4,5-ethylenedithio-1,3-dithiol-2-one 13 (750 mg, 3.60 mmol) in P(OMe)3 (12.5 mL, freshly distilled) was refluxed for 6 h. The reaction mixture was allowed to cool to room temperature. Concentration of the solution followed by column chromatography (silica gel, toluene) and recrystallization from toluene afforded 510 mg (31%) of brown needles of the ketal protected TTF 14: mp 248 °C (toluene); Rf ) 0.36 (toluene); IR (KBr) 2960, 2911, 1100, 1000 cm-1; 1H NMR (CDCl3, 200 MHz) δ 2.72 (4H, s), 3.28 (s, 4H), 4.09 (4H, s); EIMS (m/z) 456 (M.+, 100), 428 (63), 264 (90). Anal. Calcd for C13H12O2S8 (456.765): C, 34.18; H, 2.65; O, 7.01; S, 56.16. Found: C, 34.46; H, 2.72; O, 7.26; S, 54.51. 2,3-(Ethylenedithio)-5,7-(6-oxopropylenedithio)tetra-thiafulvalene (15). A solution of ketal-protected TTF 14 (200 mg, 0.438 mmol) in THF (100 mL) was acidified with concentrated sulfuric acid (10 mL) and the mixture refluxed for 18 h. After being cooled at room temperature, the solution was concentrated, taken up in CH2Cl2 (100 mL), washed with brine (2 × 100 mL), dried (MgSO4), and concentrated. The crude product was purified by column chromatography (silica gel, CH2Cl2) and recrystallization from toluene to give yellow platelets of 15 (180 mg, 100%): mp 227 °C (toluene); Rf ) 0.60 (toluene); IR (KBr) 2964, 2916, 1704 cm-1; 1H NMR (CDCl3, 200 MHz) δ 3.29 (4H, s), 3.35 (4H, s); EIMS (m/z) 412 (M.+, 87), 88 (100). Anal. Calcd for C11H8OS8 (412.712): C, 32.01; H, 1.95; O, 3.88; S, 62.16. Found: C, 31.97; H, 1.97; O, 4.66; S, 61.09. X-ray Crystallography. The X-ray measurements were carried out on crystals mounted on glass fibers using a Stoe IPDS (for 5) or an Enraf-Nonius Mach3 diffractometer (for 8 and 11) with graphite-monochromated Mo KR radiation (λ ) 0.71069 Å) at 293 K. A numerical absorption correction was applied for 5 and none for 8 and 11. The structures were solved by direct methods (SHELXS) and refined by full-matrix leastsquares (SHELXL). The hydrogen atoms were included at

6486

J. Org. Chem., Vol. 65, No. 20, 2000

idealized positions (d(C-H) ) 0.97 Å in sp3 methylenic hydrogen atoms and d(C-H) ) 0.93 Å in sp2 dCH aromatic hydrogen atoms) and not refined. Data for 5: C12H8F4S8, MW ) 484.66, red plate, monoclinic, P21/c, a ) 15.460(2) Å, b ) 4.5991(5) Å, c ) 13.0492(13) Å, β ) 109.201(13)°, V ) 876.2(2) Å3, Z ) 2, Dcalc ) 1.837 g cm-3, µ ) 1.051 mm-1, R(F) ) 0.035, wR(F2) ) 0.0564. Data for 8: C9H6S2S6, MW ) 344.50, red plate, monoclinic, P21/c, a ) 13.649(3) Å, b ) 10.996(2) Å, c )9.1740(18) Å, β ) 118.44(3)°, V ) 1306.2(4) Å3, Z ) 4, Dcalc ) 1.752 g cm-3, µ )1.042 mm-1, R(F) ) 0.0345, wR(F2) ) 0.0794. Data for 11: C11H8F2S8, MW ) 434.65, red plate, monoclinic, P21/c, a ) 13.598(1) Å, b) 8.869(1) Å, c) 14.303(1) Å, β ) 107.93(1)°, V ) 1641.2(2) Å3, Z ) 4, Dcalc ) 1.759 g cm-3, µ ) 1.094 mm-1, R(F) ) 0.0366, wR(F2) ) 0.0834.

Dautel and Fourmigue´ Crystallographic data have been deposited, in CIF format, with the Cambridge Crystallographic Data Centre and can be obtained, on request, from the Director, Cambridge Data Crystallographic Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.

Acknowledgment. We thank the CNRS and the Re´gion Pays de Loire for financial support to O.J.D. Supporting Information Available: Full X-ray crystallographic data of compounds 5, 8, and 11. This material is available free of charge via the Internet at http://pubs.acs.org. JO000518L